Resistive memory devices store information by controlling the resistance across each memory cell such that a read current through the memory cell will result in a voltage drop having a magnitude that is based on the information stored in the memory cell. For example, in certain magnetic memory devices, the voltage drop across a magnetic tunnel junction (MTJ) in each memory cell can be varied based on the relative magnetic states of the magnetoresistive layers within the memory cell. In such memory devices, there is typically a portion of the magnetic tunnel junction that has a fixed magnetic state and another portion that has a free magnetic state that can be controlled relative to the fixed magnetic state. Because the resistance through the memory cell changes based on the magnetic orientation of the free portion, information can be stored by setting the orientation of the free portion. The information is later retrieved by sensing the orientation of the free portion. Such magnetic memory devices are well known in the art.
Writing to magnetic memory cells can be accomplished by sending a spin-polarized write current through the memory device where the angular momentum carried by the spin-polarized current can change the magnetic state of the free portion. Depending on the direction of the current through the memory cell, the resulting magnetization of the free portion will either be in a first state or a second state relative to the fixed portion. In some spin torque devices, the first and second states correspond to the free portion being parallel to the fixed portion and anti-parallel to the fixed portion, respectively. If the parallel orientation represents a logic “0”, the antiparallel orientation may represent a logic “1”, or vice versa. Thus, the direction of write current flow through the memory cell determines whether the memory cell is written to a first state or a second state. Such memory devices are often referred to as spin torque transfer memory devices. In such memories, the magnitude of the write current is typically greater than the magnitude of a read current used to sense the information stored in the memory cells.
Initiating the write current or “programming current” through the memory cell may be accomplished by applying a voltage pulse across the memory cell. Because of potential defects in the structure of the magnetic tunnel junction within the cell, high programming voltages, or other reasons, the repeated application of the voltage pulses and resulting write current may cause dielectric breakdown of the tunnel barrier within the magnetic tunnel junction of the memory cell. When such breakdown or some other failure of the tunnel barrier occurs, the memory cell is no longer functional and cannot be read from or written to. As such, there is a need for robust magnetoresistive memory cells that are not as susceptible to failure in response to repeated programming operations.